This disclosure of Japanese Patent Application No. 2001-286953 filed on Sep. 20, 2001 including the specification, drawing and abstract is incorporated herein by reference in its entirety.
1. Field of Invention
The invention relates to a nonaqueous electrolyte secondary battery with increased power, and more particularly to a nonaqueous electrolyte lithium secondary battery with increased power.
2. Description of Related Art
In a rechargeable nonaqueous electrolyte lithium secondary battery, generally, positive and negative active materials are used as host solids capable of reversibly receiving and releasing lithium atoms as guest atoms. In lithium ion batteries available in the market, for example, a lithium-transition metal oxide compound such as LiMn2O4, LiNiO2 or LiCoO2 is used as a positive active material and carbonaceous coke or graphite is used as a negative active material. The electrolyte is prepared by dissolving lithium salt LiPF6 in a mixture of nonaqueous organic carbonate compounds such as ethylene carbonate, propylene carbonate, ethyl methyl carbonate, diethyl carbonate, and dimethyl carbonate. The electrolyte is impregnated in a separator.
A lithium ion battery in which lithium ions can move reversibly between electrodes with ease is possible to be charged and discharged for hundreds of cycles. The lithium ion battery, however, is likely to suffer from a capacity loss as it is repeatedly charged and discharged for a long period of time. Japanese Laid-Open Patent Publication No. 10-223258 discloses a technique for suppressing the capacity loss of the battery during charging and discharging cycles by adding a trialkoxy boroxin compound including boroxin rings to an electrolytic solution so as to prevent the capacity loss of the battery.
Besides, Japanese Laid-Open Patent Publications No. 11-3728 and 11-121033 disclose a technique for suppressing a capacity loss of the battery during charging and discharging cycles by adding a triphenyl boroxin compound including boroxin rings and its derivative compound to electrodes and an electrolytic solution. The quantity of the added triphenyl boroxin compound is 0.01 to 0.1 wt. % with respect to the weight of the electrode when it is added into the electrodes. The quantity of the added triphenyl boroxin compound is 0.01 to 0.1 mol/L with respect to the quantity of the electrolytic solution when it is added in the electrolytic solution.
When the boroxin ring compound is added to the electrolyte, the capacity loss of the battery may be suppressed during charging and discharging cycles as described above. However, power of the battery is not increased. Accordingly, the power of the battery is still not sufficient especially at a low temperature.
Also, Japanese Laid-Open Patent Publication No. 11-54151 discloses that a polymer including a boroxin ring and polyethylene oxides in its system can be used as an ion conductor. Nevertheless no description is made regarding a technique for increasing power of the battery by using an additive.
It is an object of the invention to provide a nonaqueous secondary battery, in particular a nonaqueous lithium secondary battery capable of generating increased power especially at low temperature by adding a small quantity of an additive to an electrolyte.
A nonaqueous electrolyte secondary battery includes a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, a nonaqueous electrolyte provided by dissolving a lithium salt in a nonaqueous solvent, with which the separator is impregnated. The nonaqueous electrolyte contains a compound having a boroxin ring and (poly)alkylene oxide chains, the compound being represented by a chemical formula. 
R1xe2x95x90R1xe2x80x2xe2x80x94(O-Alk1)n1xe2x80x94
R2xe2x95x90R2xe2x80x2xe2x80x94(O-Alk2)n2xe2x80x94
R3xe2x95x90R3xe2x80x2xe2x80x94(O-Alk3)n3xe2x80x94
In the above formula, AlK1, AlK2, AlK3 are identical with or different from one another, each of AlK1, AlK2, AlK3 representing one type of alkylene having a carbon number of 2 or 3, and Rxe2x80x21, Rxe2x80x22, Rxe2x80x23 are identical with or different from one another, each of Rxe2x80x21, Rxe2x80x22, Rxe2x80x23 representing one type of alkyl having a carbon number of 1 or 2.
A nonaqueous electrolyte secondary battery includes a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, a nonaqueous electrolyte provided by dissolving a lithium salt in a nonaqueous solvent, with which the separator is impregnated. The nonaqueous electrolyte contains a compound having a boroxin ring and polyalkylene oxide chains, the compound being represented by a chemical formula. 
Rxe2x95x90Rxe2x80x2(OCH2CH2)nxe2x80x94
OR
Rxe2x95x90CH3, CH3CH2 
In the formula, n represents an integer equal to or greater than 1, and Rxe2x80x2 represents an alkyl with n equal to 1 or 2.
The polymerization degree of the alkylene oxide chain in the formula is 1 to 10.
The quantity of the compound represented by a chemical formula and contained in the nonaqueous electrolyte ranges from about 0.005 to 0.3 mol with respect to 1 mol of the lithium salt contained in a nonaqueous electrolytic solution of the nonaqueous electrolyte.
The nonaqueous solvent is a mixture of at least two different nonaqueous organic carbonate solvents selected from the group consisting of ethylene carbonate, propylene carbonate, diethyl carbonate, dimethyl carbonate, and ethylmethyl carbonate.